Table ii

ABSTRACT

AN ALLOY COMPRISED MAINLY OF CHROMIUM, NICKEL AND COBALT AND EXCELLENT IN CORROSION- AND HEAT RESISTANCE, PARTICULARLY, AN ALLOY HAVING THE HIGH CORROSION RESISTANCE TO A HIGH TEMPERATURE COMBUSTION GAS CONTAINING SULFUR OXIDES AND ALKALI COMPOUNDS AND SUITABLE FOR A BLADE FOR A GAS TURBINE. THE SAID ALLOY IS PROVIDED WITH A CHROMIUM DIFFUSION LAYER ON ITS SURFACE BY A CHROMIZING TREATMENT.

Uniw Stat P e US. Cl. 416241 1 Claim ABSTRACT OF THE DISCLOSURE An alloy comprised mainly of chromium, nickel and cobalt and excellent in corrosionand heat resistance, particularly, an alloy having'the high corrosion resistance to a high temperature combustion gas containing sulfur oxides andalkali compounds and suitable for a blade for asgas turbine. The said alloy is provided with a chr0- ,mium diffusionlayeron its surface by a chromizmg treatment. l

V This application is a continuation of copending application Se'r. No. 634,900, filed May 1, 1967 and now abandomedr. r. v.

(BACKGROUND OF THE'INVENTION 1) Field at the invention The present invention relates to alloys havingexcellent heatand corrosion resisting properties to combustion .gas produced from fuels containing high sulfur, such as .C-class heavy oil which contains about 3.5% sulfur as shown in K 20051960 1 18 (Japanese Industrial Standard). More particularly, it relates to alloys showing sufiicient anti-corrosive property .in the case where such gases contain some alkali compounds, said alloy having sufficient mechanical strength at high temperatures.

(2) Description of the prior art The composition of an alloy, namely the kind of elements composing said alloy and proportions of such elements, is determined in accordance with the uses or ap plications for the alloy.

In other words, the composition of an alloy is determined according to the use so as to impartfavorable properties or performances requisite for uses of the alloys. As such properties there may be mentioned, for

instance, heat resistance, corrosion resistance, and mechanical strength (i.e. tensile strength, shearing strength, yielding value and creap strength).

The more severe the performance required for the alloy and the higher the number of performances required, the harder it is to fully satisfy those requirements.

Gas turbine blades are caused to revolve at a high velocity in contact with high temperature combustion gas.

Therefore, alloys for gas turbine blades are required to 3,664,765 Patented May 23, 1972 have three properties, i.e. sufficient mechanical strength at high temperatures above 800 0, sufficient heat resistance and sufiicient corrosion resistance to high temperature combustion gas and corrosive ash.

As alloys satisfying the above-mentioned three properties, super alloys containing metal elements such as nickel, cobalt, chromium and the like in a high quantity, for instance, Inconel, Udimet, LON-155 (refer to Table I) and others are well known. These alloys have been widely used for turbine blades.

The turbine blades made of the above-mentioned alloys are durable for a long period of time in the case where fuels containing low or no sulfur like natural gas are employed. However, the turbine blades made of the above described alloys are corroded or give rise to scales on their surfaces where fuels of high sulfur content, such as C-class heavy oil, are employed, thereby becoming frequently impossible to use in a short period of time.

The present inventors have performed numerous experiments and made studies for many years in regard to the causes of such corrosion and such scaling, and have found that:

(1) The occurrence of scales and corrosions becomes especially remarkable in the presence of sulfates of alkali metals, such as Na SO Na S O K and K S O (2) Sulfates of alkali metals are formed by reaction between sulfur oxides, such as S0 and S0 produced from combustion of S in the fuel, and alkali compounds contained in the fuel gas.

(3) A very small amount of alkali compounds is present very often in the combustion gas by the following causes:

(a) ash in the fuel, (b) dusts of alkali compounds, (0) splash of sea water.

(4) In the case Where the turbine is installed near the sea side, the effect of the above-mentioned item (0) is great, and even if alkali content in the air is in the order of 1 mg./m. or below as Na O, when sulfur oxides are present, occurrence of corrosion or formation of scales is remarkably increased.

(5) When the alkali compound is present in the form of NaCl, the action becomes particularly remarkable.

As described above, the higher the number of performauce required for the alloy and the more severe the performance required, the more difficult it is to sufiiciently satisfy the all of these requirements.

For instance, Inconel-700 possesses sufiicient mechanical strength at high temperatures, suflicient heat resistance and suflicient corrosion resistance against high temperature combustion gases and possesses the above-mentioned three performances ordinarily requisite as alloy for turbine blades:

However, Inconel-700 is not sufficiently resistant to combustion gases containing sulfur oxides and alkali compounds and is remarkably corroded in a relatively short period of time.

In addition, when said turbine blades are exposed to such a gas, scales are formed on the surface, thereby resulting in lowering the etficiency of the turbine.

It may be possible to determine the composition of an alloy so as to exhibit sufficient corrosion resistance to combustion gases containing sulfur oxides and alkali compounds. However, it is ditficult to determine the alloy composition so as 'to have such corrosion resistance,

periments up to the present that alloys satisfying the hereinbefore described five performances can beobtained by forming a chromium diffusion layer by means of chromizing method on the surface of an alloy (hereintion so as not to scale, even when exposed to combustion 5 after referred to as base alloy) composed mainly of gas containing sulfur oxides and alkali compounds, in Cr, Ni and Co and having the following composition: addition to the said four performances in total. Percent by wt Under the above-mentioned circumstances, alloys ave Cr+Ni+CO above 74 not been known which satisfy five requirements in the 13 to 2 total as described above. Ni above 1 Accordingly, fuel with high sulfur content such as C- CO above 11 class heavy oil is cheap but even when its use is eco- T '7" nomically desirable, such a fuel has not been used. The yp alloys, Which h Y the above descnbed conditions and are able to provide preferable results, are BRIEF SUMMARY OF THE INVENTION 15 Inconel-700, 5-816, S8'16B and Udimet, the Corn-p051- trons of these alloys being shown in 'Table I.

According to this invention there is provided alloys The present invention is a novel proposition basedon having five Performances as dfiscfihed ahOVethe results of investigations as described above.

Furthermore according to the present invention it is DETAILED DESCRIPTION INCLUDING also possible to make the cheap sulfur-containing fuel 00 useful as a gas turbine fuel. i PREFERRED EMBODIMENTS It is an object of the present invention to provide an Names of base alloys for accomplishing the above alloy having five performances as set forth hereinbefore. described objects and standard composition ranges of Another object of the present invention is to provide their principal constituents are shown in Table I. As can turbines blades having the above-mentioned five performbe seen from Table I, these alloys contain some by-conances. stituents in addition to Cr, Ni and Co and as can also be A further object of the present invention is to make the seen by comparing Table I with Table II, these alloys cheap sulfur-containing fuel useful as a gas turbine fuel. contain, in some cases, some by-constituents which are The present inventors have also executed laborious not shown in Table I. studies for the purpose of accomplishing the above-men- Among these by-constituents, there are those which are tiOIled j which are difficult to achieve by r contained necessarily in the steps of manufacturing the lection of alloy composit on, through alloys for and alloy (hereinafter referred to as by-constituents A) and face tr atment f alloys f tur lad those which are added in order to enhance the properties AS a result, the following f s h n m e l of alloy (hereinafter referred to as by constituents B).

The result of a corroslon t Wherem test n Further, with regard to these by-constituents, names are heated at higher temperature for a relatively short of the by-constituents-A and their composition ranges period of time in an atmosphere containing far more are giveninthe following: quantity of sulfur oxide and alkali compound as com- Percent by wt. pared with combustion gases actually used (hereinafter C below 0.4 referred to as accelerating test), is not always coinci- P below 0.03 dent with the result of a test wherein test pieces are actu- S below 0.03 ally used as a turbine blade for an extended penod of time Names and the composition ranges of the (hereinafter referred to as practical test stituents B are shown as follows. (2) Two kinds of alloys of the identical series WhlCh P ercent by wt. respectively are sub ected to the same surface treatment 40 Si below 1 5 show sometimes corrosion resistances different widely Mn below'z from each other. M 0 b e1 ow 6 For instance, an alloy formed with an aluminum dif- T1 below 4 fusion layer on the surface of Inconel-700 (called alloy- A1 below 5 1) shows a better result in the accelerating test, as com- Fe below 4 pared with an alloy formed w1th a chromium diffusion W below 8 layer on the surface of Inconel-700 (called alloy-2), Nb below 4 While in the practical test, said alloy-1 shows far inferior v I below 1 result as compared with said alloy-2. B below 0 1 Although the alloy-2 shows a good result in the pracn tical test, the alloy having a chromium diffusion layer on Further, the detailed properties of the above described the surface of InconelX (called alloy-3) shows extremealloys are described in ly bad results. Engineering Alloys (the Fourth edition), by Norman At the present stage of the investigation, it is not E. Woldman, published by Reinhold Publishing Corp. definltely evident why the above-mentioned results (1) Chapman & Hall Ltd., London, and bal in Table I and and (2) are obtained. II means a balance, i.e. percent remainder left after other However, it has been proved from the result of exconstituents have been subduced from 100. I

TABLE I Constituent Name of alloy 0 Si Mn Cr Ni Co Mo Ti Al Fe Other E21 23-34 2.5-3.5 4 48 2s 3 2.5 2.5 1 18-22 as 3.5-5 W: 3.5-8,

Nb'2.5-4. 20 Bal 4 W: 4, Nb: 4, B 0 1 Bal 11 10 1.5 Udimet500 Bal 13-20 3-5 2.5-3.2 2. 5-3.2 4 Udimet-520. Bal 12 3 2 W: l; B: 0.05 Udimet-SOO. Bal 13-20 2 5-3.2 3.8-4.6 4 B: 0.04. Udimet-700 Bal 17-20 4. 5-5.7 3-4 3.7-1.7 1 B: u.1.

A method widely known as chromizing method is utilized for forming a chromium diffusion layer on the surface of such a base alloy. The chromizing method is a method wherein the metal or alloy, that is, the substances to be treated (hereinafter simply referred to as a base alloy) is brought into contact with the vapor of CrX (wherein X is Cl, I or F), at high temperature, in a reducing atmosphere such as hydrogen gas, carbon monoxide gas or hydrocarbon or in a neutral atmosphere such as argon, to deposit Cr on the base alloy by decomposition of CrX and the thus deposited Cr diffuses and penetrates into the base alloy to form a Cr diffusion layer on the surface of the base alloy.

It is desirable to make the thickness of the Cr diffusion layer commonly to be the order of 40 to ISO 1.. The diffusion layer obtained by the chromizing method has a high Cr content on the surface and the Cr content decreases gradually towards the base alloy, and the composition of the diffusion layer approaches rapidly to the composition of the base alloy on the boundary surface of both portions. Particularly good result can be obtained when the Cr content of the diffusioh layer is above 40% and the Cr content of the surface of the said layer is from 60 to 95%.

As the chromizing methods, there are the following (1) powder method, and (2) gas method and the most widely used method is the former powder method.

(1) Powder method The powder method is a method wherein a base alloy is embedded in a mixture of finely divided powders of Cr or Cr-Fe alloy and a small quantity of NH X (wherein X is Cl, I or F) and heated in a reducing atmosphere such as a hydrogen stream. In this case, NH X reacts with fine powders of Cr or Cr-Fe alloy to produce CrX and CrX vapor thus produced comes into contact with the mother alloy to form the diffusion layer.

Further, a coagulation-preventing agent such as, for instance, kaolin or alumina is usually mixed in an amount of 0.5 to 2 times the quantity of Cr or Cr-Fe alloy powder with the Cr or Cr-Fe alloy powder in order to prevent the latter from adhering to the surface of the base alloy.

The most usually used NH X is NH Cl and about 2 to 4% of NH Cl is added to Cr or Cr-Fe alloy. The heating temperature ranges from 950 C. to 1100" C. and the heating time is in the range from 5 to hours.

A method wherein NH I is used as NH X has been developed by Diffusion Alloy Co. Ltd., England. About 0.3% of NHJ is added to Cr or Cr-Fe alloy, in which case the heating temperature is in the range from 900 C. to 950? C. and heating temperature ranges from 5 to 10 hours.

A method whereinNH F is used as NI-I X has been developed by Office National dEtudes et de Recherches Aeronautiques, France. About 10 to of NH F is added to Cr or Cr-Fe alloy, in which case the heating temperature is from 980 to 1050 C. and the heating time is from 5 to 7 hours.

(2) Gas method The gas method is a method wherein Cr or Cr-Fe alloy is caused to react with dry HCl at high temperature to produce CrCl and a chromium diffusion layer is formed by bringing the vapor of CrCl thus obtained into contact with a base alloy.

A method called BDS method has been developed as one of the gas methods, wherein CrCl vapor generated is once absorbed in alumina or the like and the base alloy is embedded in alumina, in which CrCl thus obtained has been absorbed, and then heated. The BDS method is the most practical method of the gas methods.

In the BDS method, Cr or Cr Fe alloy is mixed with porous porcelain powder, alumina or the like, through which mixture dry I-ICl gas is passed at about 1,000 C. r

strength decreases to some extent. In such a case, the me- 1 chanical strength of the base alloy can be restored by reheating the alloy' at a temperature somewhat higher than the chromizing temperature (for instance, at 1,170 C. for Inconel-700) for about 2 hours and cooling in air, and further reheating at a temperature somewhat lower than the chromizing temperature (for instance, at 820 C. for Inconel-700) for about 20 hours and cooling in air.

It is preferable to heat the said base alloy in a reducing or neutral atmosphere.

This invention is further described with reference to the following example which is illustrative but not limitative thereof.

EXAMPLE In the following Table II, names and compositions of base alloys, and chromizing methods applied to the base alloys and the results obtained when the base alloys having Cr-layer thus formed are actually used for turbine blades are shown.

:Furthermore, of chromizing methods,

P means the powder method, and G means the gas method.

The conditions of treatments, conditions of the operation of turbine are given in detail as follows:

(1) Conditions of treatment in the powder method A base alloy is embedded in a mixture of powders of alloy which is composed of 60% Cr and 40% Fe in weight of 10% of 30 to 40 mesh (Tyler sieve scale mesh), 20% of 40 to 60 mesh, 60% of 60 to mesh and 10% of 80 to mesh in size, fine alumina powder (about 30 to 60 mesh) in equal quantity of the alloy powder and 'NH Cl in the quantity of 3% of the alloy powder in weight, and then heated them in the hydrogen stream at the temperature and for the time shown in Table II.

(52) Gas method heated in the hydrogen stream at the temperature and for the time shown in Table II.

(3) The operating conditions for turbine Combustion gas at 800 C. generated from combustion of C-class heavy oil with sulfur content of 3.5% (several mg. of sodium is also contained in 1 m. of combustion gas as Na O) is used for continuous running for 10,000 hours.

In' the following Table II, by excellent is it meant that adhesion of scale and corrosion scarcely occurs; by bad is it meant that remarkable .adhesion of scale and corrosion occurs, and by fairly good is it meant that a little adhesion of scale and corrosion occurs.

Mark in the composition represents by-constituents which are not shownin Table I of standard compositions or that the analytical values of composition of alloys available on the market are slightly different from the corresponding standard composition. 

